NTU EEE scientists discover a way to create laser light on silicon

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An imaging system central to autonomous vehicles, called LiDAR (Light Detection and Ranging), will soon be able to fit into the palm of your hand thanks to the new laser invented at Nanyang Technological University School of Electrical and Electronic Engineering (NTU EEE).

NTU EEE’s new laser, made from stretched germanium nanowires, is believed to be the first silicon-compatible laser operating at a low enough operation power that can complete a toolbox for a chip-scale LiDAR.

This research, led by NTU EEE Assistant Professor Nam Donguk and NTU EEE Associate Professor Tan Chuan Seng, was recently published in Nature Communications, a peer-reviewed scientific journal under the Nature Publishing Group.

The sizes of all the present LiDAR systems are inevitably huge and their prices are costly since they comprise discrete, bulky, and expensive optical components such as lasers and photodetectors. These components enable the creation of 3D maps around a vehicle by sending highly directional light beams and detecting the reflected ones. For example, LiDAR for Google’s autonomous vehicle costs around USD 75,000 — that is even more expensive than the car itself – while its size is as large as a watermelon.

Over the past few years, there have been relentless efforts from all over the world in revolutionising LiDAR systems in size and cost by putting all the necessary optical parts on a tiny silicon chip.

However, the dream of an ultra-compact, low-cost LiDAR chip has seemed so far away because of the absence of a low-threshold laser on silicon—the most crucial component for LiDAR. Silicon-compatible materials have inherently poor light-emitting efficiency due to their indirect bandgap nature, and therefore, it has been regarded close to impossible to create an on-chip laser for LiDAR.

Researchers at NTU have invented a novel laser structure that can stretch germanium—a widely used material in silicon-based CPU chips—by a few percents just in a similar way that you stretch a rubber band. They demonstrated that the light-emitting efficiency of germanium could be dramatically improved upon stretching, thereby allowing it to emit coherent laser light under optical pumping.

As the final missing piece for chip-scale LiDAR—an efficient laser on silicon—is now discovered by this research, NTU EEE Asst Prof Nam envisions that the cost of a LiDAR system can dramatically fall to S$50 and its size may become as tiny as your finger tip.

“Our demonstration opens up a new horizon of photonic-integrated circuits by showing the feasibility of silicon-compatible lasers. Our highly stretched germanium lasers, along with other passive and active optical components, will lead to the ultimate miniaturisation of various disruptive technologies such as a tiny LiDAR chip, ultra-compact biosensors, and optical computers in your hand,” said NTU EEE Asst Prof Nam.

“Neither germanium nor silicon emits light efficiently in their natural form. Although theories in the 2000’s predicted stretching germanium to a few percents might boost its light-emitting efficiency, it has been considered impossible to lengthen a semiconductor material to such a great extent. Our team has been spending several years to experimentally realise a stretched germanium nanowire and ultimately make it lasting,” NTU EEE Asst Prof Nam explained.

How the discovery was made

For materials to emit coherent laser beams, the light should be amplified in intensity while travelling inside the material by bouncing between two optical mirrors. In un-engineered germanium, however, light intensity decreases while travelling because of larger absorption (optical loss) than amplification (optical gain).

NTU EEE Asst Prof Nam’s team first theoretically envisaged a way to obtain larger amplification than absorption to realise a germanium laser; optical gain is boosted via stretching germanium while optical loss is suppressed by lowering operation temperature.
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The team then worked with Associate Professor Tan Chuan Seng at NTU EEE, a co-leader of this research, to experimentally realise a laser structure wherein a stretch germanium nanowire is surrounded by two high-quality optical mirrors.

NTU EEE Asst Prof Nam said that the team is already investigating the further development of this laser. “Our rigorous theoretical prediction tells us we are in the right direction towards electrically-driven germanium lasers operating at room temperature that may revolutionise silicon-based technologies,” added Asst Prof Nam.